1. Field of the Invention
The present invention relates generally to a multi-channel digital amplifier, and more particularly to a multi-channel digital amplifier for shifting the phases of PWM signals, wherein the phases of the PWM signals of multiple channels, which are applied to a plurality of switching elements such as the field effect transistors of the switching circuit of the digital amplifier, are shifted to be different from each other to make the on/off times of the switching elements to be different, thus minimizing cross talk between the switching elements.
2. Description of the Prior Art
In general, a digital amplifier is a device, which pulse width modulates a digital signal having small amplitude, such as a Pulse Code Modulation (PCM) signal, to a Pulse Width Modulation (PWM) signal, amplifies the PWM signal by using an amplifying circuit having a semiconductor switching element such as a Field Effect Transistor (FET), and outputs an analog audio signal demodulated by a Low-Pass Filter (LPF) to a speaker.
FIG. 1a is a block diagram of a general digital amplifier 10. A digital input signal PCM is modulated into a low power PWM signal, and then the PWM signal is inputted to a switching circuit 14 to which high power is applied by a power supply 16 to turn on/off the semiconductor switching element of the switching circuit 14, such as a FET. The switching circuit 14 amplifies the PWM signal from low power to high power. Finally, the high power PWM signal passes through a low pass filter 18 to be demodulated to an original analog signal. This analog signal causes speaker to generate sound. This digital amplifier 10 can be integrated into a single chip because all circuits except the low pass filter 18 are digital circuits. In addition, a manufacturer of the digital amplifier can easily add, delete or modify a specific function to, from or of the digital amplifier 10 using ASIC (Application Specific Integrated Circuit) technology that is developing rapidly nowadays.
FIG. 1b is a block diagram showing a PWM converter 12 of the general digital amplifier 10 in detail. The PWM converter 12 consists of an oversampler 22 (an N times oversampler), a noise shaper 24 and a PWM modulator 26. The oversampler 22 increases a sampling frequency to change the PCM digital signal to a PWM signal with no nonlinear distortion, the noise shaper 24 shifts the frequency of a noise component to a frequency higher than an audible frequency, and the PWM modulator 26 finally converts the signal to the PWM signal.
FIG. 1c is a graph showing a PCM signal that is a digital signal to be inputted to the general digital amplifier 10, a low power PWM signal that is formed by the PWM converter 12, and a high power PWM signal that is formed by the switching circuit 14.
FIG. 1d is a graph showing the propagation delay of a PWM signal before and after amplification. The semiconductor switching element FET constituting the switching circuit 14 is not turned on/off immediately the PWM signal with low power is inputted to this circuit from the PWM converter 12, but is turned on/off after a certain time elapses. Accordingly, as shown in FIG. 1d, the switching circuit 14 outputs the high power PWM signal that has been delayed by ton and toff with respect to the low power PWM signal.
However, the digital amplifier 10 may be used as a mono type using a single channel, but in most case it is used as a multi-channel type using two or more channels. Although an overall structure of this multi-channel digital amplifier is similar to the above mentioned digital amplifier 10, there are differences between the amplifiers in that the PWM converter 12 outputs a plurality of PWM signals corresponding to the number of channels N, and the PWM signals respectively generates N high power PWM signals by N switching elements 14a, 14b, . . . , 14n, as shown in FIG. 2a. FIG. 2b shows the low power multi-channel PWM signals to be inputted to the N switching elements 14a, 14b, . . . , 14n from the PWM converter 12. However, in this conventional multi-channel digital amplifier, when the switching element 14b outputting a PWM signal of a channel No. 2 in FIG. 1b is turned on or off just before the switching element 14a outputting a PWM signal of a channel No. 1 is turned on, the delay time of the switching element 14a of the channel No. 1 is increased or decreased abnormally to distort the high power PWM signal of the switching element 14a of the channel No. 1. This phenomenon is referred to as xe2x80x9ccross talkxe2x80x9d.
In the case of a multi-channel digital amplifier that should amplify a pulse signal having a high frequency, cross talk further increases the distortion of an outputted waveform, and exerts a bad electromagnetic influence on other adjacent circuits constituting the digital amplifier 10.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a multi-channel digital amplifier, wherein the on/off times of the semiconductor switching elements of a switching circuit are made to be different from each other by shifting the phases of the low power PWM signals of each channel, thereby minimizing cross talk.
In order to accomplish the above object, the present invention provides a multi-channel digital amplifier, comprising a PWM converter for converting an input signal to N low power PWM signals (N is two or more); switching circuits for amplifying the N low power PWM signals to generate N high power PWM signals; and low pass filters converting the N high power PWM signals to analog signals inputted to a plurality of speakers; wherein the phases of the N low power PWM signals are made to be different from each other to make the on/off times of the semiconductor switching elements of the switching circuit to be different.
Preferably, the PWM converter shifts the phases of the PWM signals to make the phases of the PWM signals to be different.
In accordance with another feature of the present invention, there provided a method of manufacturing a multi-channel digital amplifier, comprising the steps of designing a digital amplification chip using Hardware Description Language, approving a design through a software or hardware method, and fabricating the multi-channel digital amplifier using the approved design.